Method of and apparatus for improved process control



Nov. 29, 1966 D, E, LUPFER 3,288,706

METHOD OF AND APPARATUS FOR IMPROVED PROCESS CONTROL Nov. 29, 1966 D. E.LUPFER 3,288,705

METHOD OF AND APPARATUS FOR IMPROVED PROCESS CONTROL 2 Sheets-Sheet 2bams A Original Filed June 12, 1961 6|] r SO 72 SI i-? Lw 7 C B 62 e3 6465 56 67 69 OB 59 Sn] 755 F/Qa United States Patent fiice :1i-,288,706Patented Nov. 29, 19616 3,288,706 METHOD F AND APPARATUS FOR IMPROVEDPROCESS CONTROL Dale E. Lupfer, Bartlesville, Okla., assignor toPhillips Petroleum Company, a corporation of Delaware Originalapplication .lune 12, 1961, Ser. No. 116,580. Divided and thisapplication Jan. 11, 1965, Ser. No.

13 claims. (ci. 20s-341) This application is a division of my pendingapplication Serial No. 116,580, filed lune 12, 1961, now U.S. Patent3,197,138.

This invention relates to an improved method of and apparatus forcontrolling a process. In one specific aspect, this invention relates toan improved method of and apparatus for controlling a natural gasolinerecovery process.

In a process control system wherein two or more variable conditionscontribute to produce a single desired result, it is conventional toanalyze the product, cornparing said analysis with a desired result. .Afeedback control is then exercised on one of the variable conditions tothus Iadjust the process in order to produce said desired result. Theprocess being controlled by a conventional feedback method of control isoften of such a dynamic nature that poor results are obtained. Processdead time frequently is the cause of inadequate control. Dead time in aprocess can be defined as the time elapsed between the initiation of achange in the process and the detection of the effect of the change uponthe process.

A typical process wherein conventional control systems are inadequate isin the :recovery `of natural gasoline. Hereinafter, the inventivecontrol system will be discussed as it specifically applies to therecovery of natural gasoline and residual gas from la raw naturalgaseous feed. But, of course, it will be understood by those skilled inthe art that the principles applied herein are equally applicable toother process systems.

A conventional natural gasoline recovery process ernploys an absorptionstep wherein lean oil Iabsorbs the natural gasoline from a raw naturalgas feed. Natural gasoline is recovered from the rich oil -by adistillation step and the stripped lean absorption oil recycled to theabsorption step. The recovered natural gasoline is then deethanized orotherwise stabi-lized. Control of the conventional natural gasolinerecovery process resides primarily in controlling the composition of therich oil stream from the absorber and the composition of the naturalgasoline recovered from the stabilizing step.

Broadly, I have discovered an improved method of and -apparatus forcontrolling a process wfherein the control system employs a shaped-pulsesignal. I have further discovered a method of and apparatus forpneumatically simulating a first order dead time approximation andproviding a shaped pulse. I have also discovered an improved method ofand `apparatus for controlling a natural gas-oline recovery processwherein the control of said natural gasoline recovery process residesprimarily in controlling the composition of the residue gas.

Accordingly, an object of this invention is to provide an improvedmethod of .and apparatus for controlling a process.

Another object of this invention is to provide a method of and apparatusfor obtaining a shaped signal pulse pneumatioa'lly.

Another object of this invention is to provide a method of and apparatusfor pneum'atically simulating a first order dead time approximation.

Another object of this invention is to provide an improved method -ofand apparatus for controlling a natural gasoline recovery process.

Another object -of this invention is to provide an improved method ofand apparatus for controlling a natural gasoline recovery processwherein the absorption step is controlled by a feed-forward orpredictive control method.

Other objects, advantages and features of my invention will be readilyapparent to those skilled in the art from the following description andlappended claims.

FIGURE l is a schematic representation of one e-mbodiment of theinventive control method.

FIGURE 2 is a schematic representation of another embodiment of theinventive method of controlling the absorber of FIGURE 1.

FIGURE 3 is a schematic representation of yet another embodiment of theinventive method of controlling the absorber of FIGURE 1.

FIGURE 4 is a schematic representation of the function of theshaped-pulse generator.

FIGURES 5a 'and 5b are diagrammatic representations of a pneumatic firstorder vdead time approximation.

FIGURES 6a and 6b are diagrammatic representations of a pneumaticshaped-pulse generator.

FIGURES 7a and 7b are diagrammatic representations yof an output pulsesignal which results from a step input change for the circuit of FIGURE4.

FIGURES 8a and 8b are diagrammatic representations of an input signaland an output signal of FIGURE 4, respectively, wherein a portion of thecircuit of FIGURE 4 has a specific transfer function.

FIGURE 9 is a diagrammatic representation of an output response ofFIGURE 4 circuit resulting from a step input change wlherein a portionIof the circuit of FIGURE 4 has yet another transfer function.

FIGURE l0 is a schematic representation of the computer of FIGURE 2.

Referring to FIGURE l, a raw natural gas feed is introduced intoabsorber 11 by means of a conduit 10. Said natural gas lfeed isfoountercurrently contacted in absorber 11 with the lean oil feed passedto said absorber 11 by means of con-duit 12 and valve 13. A rich oilstream is removed from absorber 11 and passed to a stripper 15 viaconduit 14. An overhead residual gaseous stream is removed from absorber11 by means of a. couduit 16.

An analyzing means 17, such as a chromatographic analyzer and peakreader, is provided to determine the composition of the residual gaseousstream in conduit 16. As such analyzer 17 transmits a .signal inresponse to the concentration of a constituent in the residual gaseousstream to la shaped-pulse generator 19, said shaped-pulse generator 19hereinafter more fully described. A signal is transmitted from pulse`generator 19 to a flow-recordercontroller 20 which in turn opens lorcloses valve 13 in response to lthe analysis of the residual gaseousstream in conduit 16. It is within the scope of this invention tomeasure other process variables indicative of the effectiveness of theabsorption step, such as the composition of the rich oil stream andabsorber pressure and to pass a signal representative of saidmeasurement to generator 19. Although only one absorber is hereinillustrated, it is, of course, within the skill of the 'art to controlmulti-ple absorbers operating in series, or in parallel, in a similarmanner. Control of the lean oil rate of flow is herein illustrated; itis within the scope of this invention to also control other processvariables of the absorptio step such as the raw feed rate of flow. i

Natural gasoline is separated from the absorption oil by distillation instripper 15 and removed overhead from said stripper 15 by means of aconduit 22. A lean absorption oil stream is removed from the bottom ofstripper 15 and recycled by means of conduit 21 to conduit 12.

A natural gasoline feed stream is passed -to stabilizer 23 via conduit22. Within said stabilizer 23, said natural gasoline is deethanized, orotherwise stabilized, and withdrawn from the bottom of stabilizer @23via conduit 24. A residual gaseous stream is removed from the top ofstabilizer 23 by means of a conduit 25 and passed to a means .ofcondensing said residual vapors, such .as a condenser 26. The condensedvapors are passed from condensing means 26 via conduit 28 to an overheadaccumulator 27.

A portion of the condensed overhead vapors is passed from accumulator 27to stabilizer 23 as reflux via conduit 29. The reflux rate is controlledby means of a conventional flow recorder-controller 31 opening orclosing valve 30. The remainder of the condensed vapors is withdrawnfrom accumulator 27 and recycled to said accumulator 27 via conduit 29,conduit 3:2, heat exchange means 33, and conduit 3S. At least a portionof the recycled condensed vapor-s are vaporize-d by heat exchange means33, the rate of vaporization determined by the opening or closing ofvalve 34. Vapors are passed from heat exchange means 33 to accumulator27 via conduit 35 and removed from accumulator 27 via conduit 36. Thevapors tiowing through conduit 36 are combined with the residue gasflowing through conduit 16 at the point of communication (contact zone)between conduits 16 and 36.

Stabilization within stabilizer 23 can be controlled, for example7 byopening or closing valve 43 in response to a liquid level in accumulator27, said liquid level determined, for example, by a liquid level sensingmeans 41. The signa-l is transmitted from liquid level sensing means 41to a conventional liquid level-recorder-controller 18 which in turnopens or closes valve 43 in response to the liquid level in accumulator27.

The composition of the combined residual gaseous stream in conduit 16 isde-termined by analyzing means 37, such as a chromatographic analyzer.Analyzing means 37 transmits a signal representative of the compositionto a conventional recorder-controller 38. Recorder-controller 38compares the signal received from analyzing means 37 with a set pointrepresentative of the desired composition and transmits a signal to atemperature-recorder-controller 39 which in turn opens or closes valve34 in response to the composition of the residual gaseous ow Iin conduit16.

Although not necessary to the inventive control method, an additionaladvantage -of the inventive method of control is apparent when it isdesired to combine a third gaseous stream received from a second naturalgasoline recovery process with the residual gaseous stream from absorber11 and accumulator 26 to thus blend three residual gaseous streams toproduce a combined residue gas of controlled composition, said thirdgaseous stream passed to conduit 16 by means of conduit 40.

The composition of the combined residual gaseous stream in conduit 16,in the natural gasoline recovery process lof FIGURE 1, is affected bysevere transients revolving about the absorption step of the process. Atransient can be defined as a change from a steady state operation. Asteady state operation is an operation wherein the process or .operationis maintained undisturbed. For example, a decrease in the temperature ofthe absorber lean oil feed, with all other variables remaining constant,will increase the absorption capacity of absorber 11, and thus decreasethe flow of residue gas from the absorber. Assuming that it is desirableto produce a residue gas of -constant heating value, then the heatingvalue of the combined residual gaseous stream will be reduced with thelower lean oil temperature. The temperature of the -raw natural gasfeed, and the flow 'rate of the lean oil and the natural gas raw feed,are also important in determining the heating value of the residualgaseous stream iiowing from absorber 11. The relative importance inrelation to the inventive control system will hereinafter be more fullydiscussed.

ldescribed in Cutler-Hammer Bulletin 99091.

An increase in the tlow of enriching residue gas from accumulator 27will raise the heating value of the combined residual gaseous stream inconduit 16. The heating value of the combine-d residual gaseous streamfrom the natural gasoline recovery process can, therefore, be controlledby adjusting the ow of residue gas from absorber 11 and accumulator 27with the lowest heating value obtainable determined by the heating valueo f the residue gas iiowing from absorber 11.

A disadvantage of conventional feedback control systems employed toproduce a residual gaseous stream of constant heating value is readilyapparent when `reference is made to FIGURE 1. In a conventional controlsystem, the heating value `of the combined residual gaseous streamflowing in conduit 16 would be determined by an analyzing means 37 suchas a Cutler-Hammer calorimeter In response to said determination,temperature-recorder-controller 39 would open or close valve 34, thusincreasing or decreasing the ow of enriching vapor from accumulator 27.Assuming that the temperature of the lean oil flowing to absorber 11decreases, analyzer-recorder-controller 38 would then manipulatetemperature-recorder-controller 39 to hold the heating value of thecombined -residual gaseous stream constant. Obviously, the conventionaloutlined control loop will be quite slow in reacting due to the dynamicsof the process it is controlling. It would then be necessary to operateat a higher heating value level than otherwise necessary to permit anadequate margin of safety. It is undesirable, but necessary, whenoperating a conventional feedback control system to permit the heatingvalue to vary widely from the desired control level.

Conven-tionally, the capacity of .a natural gasoline recovery process islimited by the capacity lof the absorption step. Therefore, anyadjustment made in the llow of residue gas from absorber 11 in order toproduce a desired combined residual gas having a desired heating valueshould be limited. As it is conventional to operate the .absorber orabsorbers so as to recover maximum product, it is desirable to operatewith a maximum lean oil iiow for the maximum period of time. If the owof lean oil to the absorber was adjusted in a conventional feedbackcontrol method by analyzing the combined residual gaseous stream, thelast-named objective of operating the absorber with maximum lean oilflow cannot be effectively maintained.

The inventive control system as hereinafter described provides a meansof -operating the absorption step at the maximum possible capacity whileat the same time producing a combined residual gaseous stream having aconstant heating value. Although the inventive control system ashereinafter discussed will be specilically applied to the production ofa residual gaseous stream having a constant heating value7 it is withinthe scope of this invention to apply the `inventive control system toproduce a residue gas of desired composition.

I have discovered an improved method of controlling the ow of residuegas in a natural gasoline recovery process wherein a change in theheating value of the residue gas flowing from absorber 11 is noted byanalyzing means 17. A signal is transmitted from analyzing means 17 to aseries of controls to adjust the ow of lean oil to absorber 11 inresponse to the determination of analyzing means 17. A continuous signaltransmitted from analyzing means 17 is caused to decay by means of ashapedpulse generator 19, thereby returning the rate of lean oil flow tothe optimum level prior to a further change inthe heating value of theresidue gas from absorber 11 as noted by analyzing means 17. The controlof the combined residual gaseous stream liowing in conduit 16 iscompleted by determining the heating value ofthe combined residualgaseous stream with analyzing means 37 and increasing or decreasing theflow of residue gas lfrom accumulator 27 in response to saiddetermination. The inventive control system thus provides a means forrapidly adjusting the natural gasoline recovery process in response to achange occurring in the absorption or stabilizing steps to produce acombined residual gaseous stream of constant heating value.

It can readily be seen that the flow of residue lgas from accumulator 27can also be adjusted to produce a combined residual gaseous stream ofconstant heating value with a change in the heating value of a thirdfeed stream passed to conduit 16 via conduit 4i) and noted by analyzingmeans 37. With the possible occurrence of rapid changes in theabsorption step as occasioned by a change in the lean oil temperature,raw gaseous feed temperature, or raw gaseous feed flow rate, forexample, the inventive control system will act to control the heatingvalue of the absorber residual gaseous stream in such a manner that thechange in the heating value of the residual gaseous stream flowing fromabsorber 11 Will occur slowly. rIhis permitsanalyzer-recorder-controller 38 the time necessary to finally controlthe heating value of the combined residual gaseous stream by adjustingthe flow of the residue gas from accumulator 27.

The shaped-pulse generator 19 transmits a signal initially undisturbedfrom analyzer 17 to oW-recorder-controller 20. Thereafter, shaped-pulsegenerator 19 causes the signal to fade, thus causingflow-recolder-controller 2@ to return valve 13 to the normal operatingposition ernployed prior to a subsequent change in the residue gas. Thenormal operating position is the position of valve 13 prior to theinitial change in the residue gas heating Value from absorber 11detected by analyzing means 17. A shaped-pulse generator thus operatesso that as each signal representative of a heating value change isapplied to the input of the shaped-pulse generator, the output of theshaped-pulse generator will respond by first transmitting a signalrepresentative of the heating value change. This output signal ismaintained for a desi-red period `of time and then permitted to returnback to the original value that existed before the input signal to theshaped-pulse generator was changed. This permits the actual change inthe heating value content of the residual gas flowing from the absorber11 to occur slowly.

In order to more fully explain the operation of the shaped-pulsegenerator, reference is made to FIGURE 4. The circuit illustrated inFIGURE 4 can, for example, be employed to yield a square-pulse signalfor a step signal input as illustrated by FIGURES 7a and 7b, Where C iserst, a transfer function in the Laplace domain of pure dead time ortransport delay; Si(A) is an input signal; Sc is an output signal; t istime; S is the Laplace operator. The overall transfer function for thecircuit of FIGURE 4 can be written as:

If the block shown in FIGURE 4, for example, has a transfer function ofG, G being the transfer function of a rst order lag, a response to a`step input signal can be graphically illustrated by FIGURES 8a and 8b,where FIGURE 8a represents the input to the summing junction D andFIGURE 8b represents the output pulse. The overall transfer function forthe circuit of FIGURE 4 then becomes:

1So l TIS ,Si-1 GFl TlS-l-l- TlS-l-z' where T1 is the time constant ofthe first order lag.

If the C block shown in FIGURE 4 has a transfer function of Ge-St, whereG is the transfer function of a third order lag and e-St is again puredead time, the response to a step signal input will be as illustrated byFIGURE 9. FIGURE 9 represents the theoretical response desired. Thiscontrol system, however, is difficult to obtain in a phi-actical matterbecause of the process dead time. For this reason an approximation ofpure dead time is em- 6 ployed. Dead time can be approximated with thetransfer function:

Tzs- 1 T215' -j- 1 where T2 is the time constant of the lirst order lagin the above approximation. This is known as a first order dead timeapproximation. A control system with this transfer function will act aspure dead time within certain frequency limitations. These limitationsare set by:

where W is the frequency in radians/minute and t is the desired deadtime in minutes.

Thus, if it is desired to have a six minute dead time, a frequency of0.1 radian per minute cannot :be exceeded. At the maximum allowablefrequency for a given dead time, there will be an error of one degree inthe theoretical phase shift for pure dead time. If the allowablefrequency is exceeded, the phase shift error will exceed one degree.When the study of a process indicates that certain frequencies cannot bepassed, then it is possible to employ dead time approximation toadvantage.

I have discovered a method of and apparatus for pneumatically simulatinga first order dead time approximation with a transfer function of:

Where g is the adjustable gain of the computing relay; C and B are inputvariables. Input signal Si is also transmitted to computing relay 50 viaconduit 54 and restriction means 51. In the derivation of the transferfunction for the pneumatic circuit of FIGURE 5a, the following equationscan be Written:

where T2 is the time constant and is equal to the resistance times thecapacitance. In the derivation of pressure B, it is noted that the rateof change of pressure B with respect to time is proportional to the netflow of gas into or out of the volume downstream of the restrictionmeans, and is inversely proportional to the Volume capacitance C(lbs./p.s.i.). Therefore:

where W1=ilow of gas (lbs/sec). The ow of gas through the restrictionmeans is proportional to the pressure drop (Si-B) .and inverselyproportional to the resistance value R (p,s.i./lb./sec.). Therefore:

Si-B

Substituting this expression into the rst equation to eliminate W1 thereis obtained:

d 1 5' Si B da RC' where RC is the time constant T2 g1g Si B di -T T2 7Using operation notation, substitute S for d/ dt to obtain:

Si-B Si BS= T2 or B-- -T2S i l The resistance is determined by measuringthe pressure drop across restriction means or restrictor 51 and dividingsaid pressure drop by the quantity of flow through said restrictor 51,and the capacitance is the change in quantity per unit change inpressure in bellows B. Substituting in the equation for computing relay50, there is obtained:

Si Si SF+r2s+1 The transfer function for the pneumatic circuit of FIG-URE Sa then becomes:

S -T2S-l+2 1/(T2Sl 1ST," 2(T,s+1) f2 T2s+1 This, as previously noted, isthe proper form of the desired transfer function for a circuitsimulating a iirst order approximation of process dead time. It is notedthat the steady state gain of the pneumatic -computing relay isone-half. However, this is not important so long as the form is correct.

I have discovered a method of and apparatus for pneumatically obtaininga shaped-pulse generator. Referring to FIGURE 6a, there is shown twopneumatic computing relays, 60 and 72, such as the Foxboro M56-1 addingrelay illustrated in Technical Information Bulletin 37-A- 57Adistributed by Foxboro Company, Foxboro, Massachusetts. Each ofthe saidrelays is capable of solving the following equation:

Where g is the adjustable gain of the relay; A, B and C are inputvariables. An input of relay 60 is not employed. Relay 60 must then becapable of solving the equation:

An input pneumatic signal Si is transmitted to computing relay 72 bymeans4 of a conduit 61. Input signal Si is also transmitted to computingrelay 60 via conduits 68 and 69 passing through restriction means 62,volume 63, restriction means 64, volume 65, restriction means 66, andvolume 67. In addition thereto, the pneumatic signal passing as an inputB to computing relay 60 passes through restriction means 70. Computingrelay 60 transmits a signal via conduit 73 to computing relay 72 as aninput variable B. As previously noted, computing relay 60 andrestriction means 70 constitute a means of providing a first order deadtime approximation.

Restriction means 62, 64, 66 and volume means 63, 65 and 67 will havethe response of a third order exponential lag. Assuming that the thirdorder lag portion of the pneumatic circuit has a transfer function G andincorporating the transfer function of the pneumatic circuit of FIGUREa, the following equations can be written for computing relay 60:

computing relay 60, the following equation is obtained:

S I StG ai FTS-H 2 The following equations can be Written for computingrelay 72:

A=Bias pressure set to operate pulse at desired level.

se sa B=SFTS+1 T The above values can be substituted into the equationfor computing relay 72 to obtain:

where g equals one-half. The pneumatic circuit of FIG- URE 6a With theabove transfer function will respond to a signal step input lchange asillustrated by FIGURE The shape of the decay `in the output signal isdependent upon the third order exponential lag. It is, of course, withinthe scope of this invention to employ other type lags, depending uponthe dynamics of the controlled process. For example, the arrangement ofrestriction means 62, 64, 66 and volume 63, 65 and 67 result in what isconventionally known as a third order interacting exponential lag. Thesecan, if the process dictates, be second order, fourth order, etc.Non-interacting lags can also be utilized. For a non-interacting lag,each individual lag element (restriction means 62 and volume 63constitute a single lag) can be isolated with a conventional isolationrelay.

Referring to FIGURE 5b, there is illustrated another circuit capable ofsimulating a rst order dead time approximation pneumatically. An inputpneumatic signal Si is transmitted via conduit to a conventionalpneumatic computing relay, such as the Foxboro M56-1 adding relaycapable of solving the following equation:

Output=A -C-l-B Where A, C and B are input variables. Input signal Si isalso transmitted to computing relay 81 via conduit 82, restriction means83, and conduits 84 (input variable B) and 85 (input variable A). Anoutput signal is transmitted from computing relay S1 via conduit 86. Inthe derivation `of the transfer function for the pneumatic circuit ofFIGURE 5b, the following equations can be written:

Substituting in the equation for the computing relay 81 there isobtained:

S i S i 'S0- T2S-f-1 S`+T2Sl The transfer function for the pneumaticcircuit of FIG- URE 5b then becomes:

TQS- 1 The above is the rst order dead time approximation exactly of thesame form as that obtained `by the circuit of FIGURE 5a with theexception that the circuit of FIGURE 5b does not have a steady stategain of one-half. When employing the pneumatic circuit of FIGURE 5b,

the shaped-pulse generator will appear as illustrated in FIGURE 6b.

Referring to FIGURE 6b, there is shown two pneumatic relays, and 91.Each of said computing relays is capable of solving the followingequation:

Output=A -C-l-B 9 `9i) via cond-uit 100, restriction means 101 andconduit 102 as input variable B and as input variable A via conduit 103.An output pneumatic signal Sn is transmitted from computing relay 90 viaconduit 104 to computing relay 91 as an input variable A. An outputsignal is transmitted from relay 91 via conduit '105.

Restriction means 94, 96, 98, and volume means 95, 97, and 99 will havethe res-ponse of a third order exponential lag. If, as in the case ofFIGURE 6a, it is ass-umed that the third order lag portion of Ithepneu-matic circuit has a transfer function G, and incorporating thetransfer function of a pneumatic cir-cuit of FIGURE b, the followingequations can be written for computing relay 90:

Substituting the above values into a general equation for computingrelay 90, the following equation is obtained:

The following equations can be written for computing` relay 91:

B=bias Ipressure SG TS -il Substituting the above values into theequation for relay 91, there is obtained:

The pneumatic circuit of FIGURE 6b, with the above transfer functionwill respond to a signal step input change as illustrated by FIGURE 9.

FIGURE 6b will provide the same desired pulse shape as FIGURE 6a `withthe exception that FIGURE 6b does not have a gain of one-half. Foreither the circuit of FIGURE 6a or FIGURE 6b, the fixed bias adjustmentof relays 72 and 91, respectively, sets the level about which the pulsesystem operates.

Referring to FIGURE 2, there is illustrated another embodiment of theinventive control method. C-ontrol of the absorber 11 can be conductedon a feed-forward basis, the remainder of the control system is asillustrated in FIGURE 1. The temperature and ratey of ow of the lean oilfeed, and the temperature and rate of flow of the natural lgas raw feedcan be measured and the results transmitted to a computer 46. Anempirical equation can be developed which relates to the fuel lfeed gasflow, feed gas temperature, lean oil ow and lean oil temperature to theabsorber 11 residue gas composition or heating va'lue. Central computer46 can then be employed to provide a continuous computed va-lue of thecomposition of the residue gas as an input signal into shaped-pulsegenerator 19. By predicting the composition or heating value of theresidue gas in this manner, and by opening or closing valve 13 inresponse to said prediction, a more rapid control response is obtained.

The empirical equation for computer 46 can be written as:

Y=heating val-ue of residue gas TG=temperature o'f raw feedTLztemperature of ilean oil FG=rate of `-flow of raw feed A differentialpressure representative of the rate of ow aassfr Although the equationhas [been written so as to determine the heating value of the residue,it can also be ernployed to Idetermine the concentration of a particularresidue gas constituent. For example, if it is desired to absorb Cs andheavier hydrocarbons in an absorption zone, the above equation ca-n beemployed to determine the concentration (Y) of C4s in the residue gas.

As the rate of change of Y with respect to changes in TL is not linear;the above equation becomes:

where e and f are constants determined empirically.

A computer capable -of solving the a'bove equation is illustrated inFIGURE 10. Referring to FIGURE 10, the temperature TL of the -lean oilfeed to the absorber is measured by a temperature sensing means notherein illustrated. A signal representative of said temperaturemeasurement is transmitted by transmitter 110 via conduit 111 t-o ameans 112 of squaring an input variable. If, for example, thetemperature sensing means employed is a thermocouple, and squaring means112 is a pneumatic instrument, transmitter 110 can be any commerciallyavailable instrument which will transpose an electrical input signalinto a pneumatic output signal, such as a Minneapolis Honeywell ElectrikTel-O-Set MV/P transmitter described in Minneapolis Honeywell Catalog FS1003-211.

Squaring means 11.2 is -an instrument capable of squaring an inputIvari-able and multiplying the result obtained by a constant. An:instrument capable orf performing this function lis the Sorteberg forceIbridge described by Minneapolis-Honeywell Regulator Company,Philadelphia, Pennsylvania in Catalog C 1 dated December 1958. A signalis transmitted from squarer 112 to a totalizer 1114 rvia conduit means113.

A signal representative of said temperature measurement is alsotransmitted wia conduits 1'111 Iand 115 lto a totalizer 1,14 as input C,said tot-alizer -capable of solving the equation:

Output=AC+B where A, B .and C are variable inputs of the totalizer. Aspreviously noted, an instnurnent capable lof performing this operationis the Foxboro M56-1 adding relay. As in the case of the lean oiltemperature, the temperature TG of the raw natural gaseous feed ismeasured and a signal .representative of said temperature measurement istransmitted by a conventional transmitter 116 via cond-uit means 1117 tototalizer 114 as variable input A. Totalizer 1-114 transmits a signalvia conduit means 123 to a totalizer l122 Ias input B. Totalizer y122 isan instrument capable of performing the salme 'function as totalizer114.

A flo-w sensing means such as yan loriice across which a pressuredifferential is developed is placed in the conduit through which thelean oil is passed to the absorber.

squared is transmitted by a :conventional differential pressuretransmitter 41118 to a square root extractor '120 lvia conduit 119.Sq-uare `root extractor 120 is a commercially available instrumentcapable olf extracting the square root of an input variable, multiplyingthe result obtained by a constant and transmitting the result tototalizer 122 as input C 'via conduit means 121.

In the same manner the rate of flow of the raw natural gaseous feed tothe absorber is determined and a differential pressure .representativeof the square of the rate of flow is transmit-ted by a differentialpress-ure transmitter 1124 to square r-oot extractor 126 via conduit125. Square root extractor 126 must be capable of penforming the sainefunction as square root extractor `1'20 and transmitting a resultingsignal to totalizer 122 as an input A via conduit means .127.

Totalizer 122 thus will transmit a signal via conduit means 128 which isrepresentative of the predictive cornposition or heating value of theabsorber residue gas. It is Within the scope of this invention to employa conventional electronic computer to perform the functions of computer46 of FIGURE 2 in the inventive control system.

Assuming that the maximum changes in the feed 'gas temperature and floware slow and are of such a quantity as to cause only a slow deviation inthe residual gas composition, another embodiment of the inventivecontrol method is available. The lean oil ilow can then Ibe manipulatedas a function of the lean oil temperature in a predictive manner asillustrated in FIGURE 3. A signal is transmitted to a multiplier 47,said signal proportional to the lean oil temperature. Multiplier 47multiplies the temperature of the lean oil rfeed by a constant (K) sothat the output of multiplier 47 is of lanapropriate magnitude to supplyan input to pulse generator 19. The lean oil ow must Ibe adjusted tocompensate for a change -in the lean oil temperature. The signal istransmitted from multiplier 47 t-o shaped-pulse generator 19, and fromshaped-pulse generator 19 in the manner heretofore described.

In order to demonstrate operating lfeatures of the inventive controlsystem, the following examples are presented as illustrative.

Example I The inventive control method illustrated by FIGURE 2 wasemployed to control the heating value of the residue ygas flowing fromabsorber 11. The raw natural .gas feed to absorber 11 was of thefollowing composition:

Mol percent Nitrogen 15.16 Methane 72.06 Ethane 6.12 Prop ane 4.12B-utanes 1.70 Pentanes .5 6 Hexane .28

where Y=heating value of residueigas in B.t.u. 'per cubic footTG=temperature of feed gas, F.

T L=temperature of lean oil, F.

FG=iiow of ffeed gas, cubic ft./ day FL=flo w of lea-11 oil, |gal./day

Computer 46 was set to solve the .a-bove equation, trans- Example II Thefunction of the shaped-pulse generator 19 or FIGURE 1 is illustrated bypassing a natural gas feed 12 to absorber |1=1, said natural gas feedhaving the Asame composition as in Example I. The residue gas owing fromabsorber 11 is of the following composition:

Mol percent Nitrogen 15 .45 Methane 73.45 Ethane 6.24

Propane 3.82 Butanes 1.02

Pent-anes .02

A chromatographic analyzer 17 determines the concentration of propane inthe residue and transmits a 9 -p.s.i.g. pneumatic pressurerepresentative of this concentration to shaped-.pulse generator 19.Shaped-pulse generator in turn transmits a 15 ip sig. pneumatic pressureto so position valve '13 to permit maximum lean oil (mineral seal oil)flow to absorber 11. The temperature o-f the lean oil is 70 F. y

A sudden decrease in the temperature of the lean oil to 60 F. reducesthe concentration of propane in the residue gas flowing from absorber11. This change is noted by chromatographic analyzer 17. Analyzer 17transmits a signal less than 9 p.s,i.g. to shaped-pulse generator 19which in turn transmits a signal to valve 13, thus reducing the ow oflean oil'to absorber 11. After a period of 35 minutes, the rate of iiowof lean oil to absorber 11 is returned to the maximum and analyzer 17 1stransmitting a constant pneumatic pressure of 8 p.s.i.g. representativeof a concentration of 2.82 mol percent of propane in the residue gas.

Operating in the above manner provides the necessary time (35 minutes)during which the flow of enriching gas from accumulator 27 can beincreased to maintain a cornbined residue gas having the desiredconcentration of propane.

Operating Without controlling the ow of lean oi1 to absorber 11 in theinventive manner results in the concentration of propane in the residuegas iiow from absorber 11 leveling out at 2.82 mol percent after aperiod of 10 minutes, following the decrease in the lean oiltemperature. The period of time in Which an adjustment in the flow ofenriching gas can be made is reduced thereby decreasing theeffectiveness of the control system.

It is within the scope of this invention to electronically provide ashaped-pulse by conventional methods and to employ the electronicallygenerated shaped pulse in the inventive control system.

It is within the scope of this invention to adjust two process variablesin response to a measured process variable wherein the adjustmentapplied to one of the process variables is removed after a period oftime with nal control of the process residing in the adjustment of these-cond process variable in response to the measured variable. Thismethod of control is particularly adapted to the control of processeswherein a rapid process response is obtained by an adjustment of theprocess variable from which the adjustment is removed after a period oftime.

As will -be evident to those skilled in the art, various modificationsof this invention can be made, or followed, in the light of theforegoing disclosure andv discussion without departing from the spiritor scope thereof.

I claim:

1. A method of controlling a process which comprises measuring a firstinput process variable of said process, forming and transmitting a firstsignal C representative of said first process variable measurement to acomputing zone, applying said first signal to the inlet of a restrictionzone to produce at the outlet of said restriction zone a second signalB, passing said second signal to said computing zone, said computingzone solving the equation Output=g(-C) -l-B where g is the adjustablegain of said computing zone, said i3 computing zone and said restrictionzone having a transfer function equivalent to (T-Si where T is the timeconstant of a first order lag and S is the Laplace operator, and passinga third signal representative of said Output from said computing zone toa means for manipulating a second process variable of said processresponsive to said third signal and thereby manipulating said secondprocess variable.

2. A method of controlling a process which comprises measuring a firstinput process variable, forming and transmitting a first signal Crepresentative of said first process variable measurement to a computingzone, applying said first signal to the inlet of a restriction zone toproduce at the outlet of sai-d restriction zone a second signal B,passing said second signal to said computing zone, said computing zonesolving the equation where g is the adjustable gain of said computingzone, said computing zone and said restriction zone having a transferfunction equivalent to TS-l TS+1 where T is the time constant of a firstorder lag and S is the Laplace operator, passing a third signalrepresentative of said Output from said computing zone to a means formanipulating a second input process variable of said process to therebymanipulate said second input process variable responsive to said thirdsignal, measuring `an output process variable representative of theeffectiveness of said process, and forming and transmitting a signalrepresentative of said output process variable measurement to a meansfor manipulating a third input process variable to thereby manipulatesatid third input process variable responsive to said output processvariable measurement.

3. In a process which comprises passing a raw natural gas feed to anabsorption zone, passing a lean absorption oil feed to said absorptionzone, withdrawing from said absorption zone a residue gas, andwithdrawing from said absorption zone a rich absorption oil containingnatural gasoline; a method of control which comprises measuring thetemperature of said lean absorption oil feed, forming and transmitting afirst signal C representative of said measurement to a Computing zone,applying said first signal to the inlet of a restriction Zone to produceat the outlet of said restriction zone a second signal B, passing saidsecond signal to said computing zone, said computing zone solving theequation where g is the adjustable gain of said computing zone, said-computing zone and said restriction zone havlng a transfer functionequivalent to where T is the time constant of a first order lag :and Sis the Laplace operator, and passing a third signal representative ofsaid Output from said computing zone to a means for manipulating therate of fiow of lean absorption oil feed to said absorption zoneresponsive to said third signal and thereby manipulating said iiow rate.

4. In a nat ural gasoline recovery process which comprises contacting `anatural gas feed with a lean absorption oil field in an absorption zone,withdrawing from said absorption zone a first residue gas, passing varich absorption oil from said absorption zone to a separation zone,withdrawing lean absorption oil from said separation zone, passing anatural gasoline containing stream from said separation zone to astabilization zone, withdrawing from said stabilization zone astabilized natural gasoline product, and withdrawing from saidstabilization zone a second residue gas; a method of control whichcomprises combining said first residue gas and said second residue gasin a contact Zone, measuring a first output process variable of saidabsorption Zone, forming and transmitting a first signal Crepresentative of said measurement to a computing zone, applying saidrst signal to the linlet of a restriction zone to produce at the outletof said restriction zone a second signal B, passing said second signalto said computing zone, said computing Zone solving the equation where gis the adjustable gain of said computing zone, said computing zone andsaid restriction zone having a transfer function equivalent to TS -llwhere T is the time constant of a first order lag and S is the Laplaceoperator, and passing a third signal representative of said Output fromsaid computing zone to a means for manipulating a first inputprocessvariable of said absorption zone responsive to said third signal,measuring a property of said combined residue gas which isrepresentative of the composition thereof, and manipulating the rate offiow of said second residue gas to said contact Zone in response to saidcomposition measurement of said combined residue gas.

5. In a natural gasoline recovery process which comprises contacting anatural gas feed with a lean absorption oil feed in an absorption zone,withdrawing from said absorption zone :a first residue gas, passing arich absorption oil from said absorption Zone to a separation Zone,withdrawing lean absorption oil from said separation zone, passing anatural gasoline containing stream from said separation zone to astabilization zone, withdrawing from said stabilization zone astabilized natural gasoline product, and withdrawing from saidstabilization Zone a second residue gas; a method of control whichcomprises combining said first residue gas and said second residue gasin a contact zone, measuring a property of said first residue gas whichis representative of the composition thereof, forming and transmitting afirst signal C representative of said measurement to a computing zone,applying said first signal to the inlet of a restriction zone to produceat the outlet of said restriction zone a second signal B, passing saidsecond signal to said computing zone, said computing zone solving theequation where g is the adjustable gain of said computing zone, saidcomputing zone and said restriction zone having a transfer functionequivalent to w Ts-i Ts+1 where T is the time constant of a first orderlag and S is the Laplace operator, and passing a third signalrepresentative of said Output from said computing zone to a means formanipulating the rate of fiow of lean absorption oil feed to saidabsorption zone responsive to said third signal, measuring a property ofsaid combined residue gas which is representative of the compositionthereof, and manipulating the rate of flow of said second residue gas tosaid contact zone in response to said composition measurement of saidcombined residue gas.

6. The method of control -of claim 5 wherein a third residue gas isintroduced into said contact zone.

7. Apparatus comprising means for measuring a first process variable, afirst pneumatic computing means, means for forming a first pneumaticpressure signal C representative of said first process variablemeasurement, first conduit means communicating with said first computingmeans fo'r transmitting said pneumatic pressure C representative of saidfirst process variable measurement from said means for forming to saidfirst computing means, a lag means, second conduit means communicatingbetween said first conduit means and said lag means, a second pneumaticcomputing means, third -conduit means communicating between said lagmeans and said second computing means for transmitting a pneumaticpressure C to said second computing means, a restriction means, fourthconduit means communicating between said lag means and said restrictionmeans, fifth conduit means communicating between said restriction means#and said second computing means for transmitting a pneumatic pressureB' to said second computing means, said second computing means solvingthe equation where g is the adjustable gain of said second computingmeans, sixth conduit means communicating between said second computingmeans and said first computing means for transmitting a pneumaticpressure B representative of said Output' to said first computing means,means for transmitting a bias pressure A to said first computing means,said first computing means solving the equation means for manipulating asecond process variable, and seventh conduit means in communication withsaid first computing means for transmitting a pneumatic pressurerepresentative of said Output from said first computing means to saidmeans for manipulating a second process Variable to thereby manipulatesaid second process variable responsive to said Output.

8. Apparatus comprising means for measuring a first input processvariable, a first pneumatic computing means, means for forming a firstpneumatic pressure signal C representative of said first processvariable measurement, first conduit means in communication with saidiirst computing means for transmitting said pneumatic pressure Crepresentative of said first process variable measurement from saidmeans for forming to said first computing means, a lag means, secondconduit means communicating between said first conduit means and saidlag means, a second pneumatic computing means, third conduit means-communicating between said lag means and said second computing meansfor transmitting a pneumatic pressure C to said second computing means,a restriction means, fourth conduit means communicating between said lagmeans and said restriction means, fifth conduit means communicatingbetween said restriction means and said second computing means fortransmitting a pneumatic pressure B to said second computing means,sixth conduit means communicating between said restriction means andsaid second computing means for transmitting a pneumatic pressure A tosaid second computing means, said second computing means solving theequation seventh conduit means communicating between said secondcomputing means and said first computing means for transmitting apneumatic pressure A representative of said Output' to said firstcomputing means, means for transmitting a bias pressure B to said firstcomputing means, said first computing means solving the equationOutput=g(A -C) -i-Bl where g is equal to the adjustable gain of saidfirst computing means, means for manipulating a second input processvariable, and eighth conduit means in communication with said firstcomputing means for transmitting a pressure representative of saidOutput from said first computing means to said means for manipulating asecond input process Variable to thereby manipulate said second inputprocess variable responsive to said Output.

9. Apparatus comprising means for measuring a first input processvariable, means for forming a first pneumatic pressure signal Crepresentative of said first process variable measurement, a pneumaticcomputing means,

first conduit means in communication with said computing means fortransmitting said first pneumatic pressure signal from said means forforming to said computing means, a restriction means, second conduitmeans cornrnunicating between said first conduit means and saidrestriction means, third conduit means communicating between saidrestriction means and said computing means for transmitting a pneumaticpressure B to said computing means, said computing means solving theequation Output=g(-C) +B where g is the adjustable gain of saidcomputing means, said computing means and said restriction means havinga transfer function equivalent to TS-1 T S 1 where T is the timeconstant of a first order lag and S is the Laplace operator, means formanipulating a second input process variable, fourth conduit means incommunication with said computing means for transmitting a pneumaticpressure representative of said Output from said computing means to saidmeans for manipulating said second input process Variable to therebymanipulate said second input process variable responsive to said Output,means for measuring an output process Variable representative of theeffectiveness of said process, means for manipulating a third inputprocess variable, and means for forming and transmitting a signalrepresentative of said output process variable measurement to said meansfor manipulating a third input process variable.

10. In an absorption process which comprises passing a raw natural gasfeed to an absorption zone, passing a lean absorption oil feed to saidabsorption zone, withdrawing from said absorption zone a residue gas,and withdrawing from said absorption zone a rich absorption oilcontaining natural gasoline; a method of control which comprisesmeasuring a property of said raw natural gas feed representative of therate of flow of said raw natural gas feed to said absorption zone,forming and transmitting a first signal representative of said rawnatural gas feed rate measurement to a computing zone, measuring aproperty of said raw natural gas feed representative of the temperatureof said raw natural gas feed, forming and transmitting a second signalrepresentative of said temperature measurement to said computing zone,measuring a property of said lean absorption oil feed to said absorptionzone representative of the rate of flow of said lean absorption oilfeed, forming and transmitting a third signal representative of saidlean absorption oil feed rate measurement to said computing zone,measuring a property of said lean absorption oil feed representative ofthe temperature thereof, forming and transmitting a fourth signalrepresentative of saidlean absorption oil feed temperature measurementto said computing zone, said computing zone solving the equation where Yis equal to the predicted heating Value of said residue gas, TG is equalto the temperature of said raw natural gas feed as represented by saidsecond signal, TL is equal to the temperature of said lean oil asrepresented by said fourth signal, FG is equal to the rate of flow ofsaid raw natural gas feed as represented by said first signal, FL isequal to the rate of ow of said lean oil feed as represented by saidthird signal, K is a constant, and a, b, c and d are constantsrepresentative of the rate of change of Y with changes in TG, TL, FG andFL, respectively, forming and transmitting a first pneumatic pressuresignal C from said computing zone representative of said predictedheating value of said residue gas to a second computing zone, applyingsaid first pneumatic pressure signal to the inlet of a restriction zoneto produce at the outlet of said restriction zone a second pneumaticpressure signal B, passing said second pneumatic pressure signal to 17said second computing zone, said second computing zone solving theequation where g is the adjustable gain of said second computing zone,said second computing zone and said restriction zone having a transferfunction equivalent to where T is the time constant of a first order lagand S is the Laplace operator, and passing a third pneumatic pressuresignal representative of said Output from said second computing zone toa means for manipulating the rate of flow of lean absorption oil feed tosaid absorption zone responsive to said third pneumatic pressure signal.

11. In an absorption process which comprises passing a raw natural gasfeed to an absorption zone, passing a lean absorption oil feed to saidabsorption zone, withdrawing from said absorption Zone a residue gas,and withdrawing from said absorption zone a rich absorption oilcontaining natural gasoline; a method of control which comprisesmeasuring a property of said raw natural gas feed. representative of therate of flow of said raw natural gas feed to said absorption zone,forming and transmitting a rst signal representative of said raw naturalgas feed rate measurement to a computing zone, measuring a property ofsaid raw natural gas feed representative of the temperature thereof,forming and transmitting a second signal representative of saidtemperature measurement of said raw natural gas feed to said computingzone, measuring a property of said lean absorption oil feedrepresentative of the rate of flow of said lean absorption oil feed. tosaid absorption zone, forming and transmitting a third signalrepresentative of said lean absorption oil feed rate measurement to saidcomputing zone, measuring a property of said lean absorption oil feedrepresentative of the temperature thereof, forming and transmitting afourth signal representative of said lean absorption oil feedtemperature measurement to said computing zone, said computing zonesolving the following equation where Y is the predicted concentration ofa constituent of said raw natural gas feed in said. residue gas, TG isthe temperature of said raw natural gas feed as represented by saidsecond signal, TL is the temperature of said lean absorption oil feed asrepresented by said fourth signal, FG is the rate of iiow of said rawnatural gas feed as represented by said first signal, FL is the rate offlow of said lean absorption oil feed. as represented by said thirdsignal, K is a constant, a, b, c and d are constants representative ofthe rate of change of Y with changes in TG, TL, FG and FL, respectively,and where e .and f are constants determined empirically, forming andtransmitting a first pneumatic pressure signal from said computing zonerepresentative of said predicted concentration of said constituent insaid residue gas to a pneumatic computing zone, applying said firstpneumatic pressure signal to the inlet of a restriction zone to produceat the outlet of said restriction zone a second pneumatic pressuresignal B, passing said second pneumatic pressure signal to saidpneumatic computing Zone, said pneumatic computing zone solving theequation Output=g(-C) +B where g is the adjustable gain of saidpneumatic computing zone, said pneumatic computing zone and saidrestriction zone having a transfer function equivalent to is the Laplaceoperator, Vand passing a third pneumatic pressure signal representativeof said Output from said pneumatic computing zone to a means formanipulating the rate of flow of lean absorption oil feed to saidabsorption zone responsive to said third pneumatic pressure signal.

12. Apparatus comprising an absorber, a stripper vessel, a stabilizervessel, first conduit means in communication with the lower region ofsaid absorber, second conduit means in communication with the upperregion of said absorber, third conduit means communicating Abetween thebottom of said absorber and said stripper vessel, fourth conduit meanscommunicating vbetween the upper region of said stripper vessel and saidstabilizer vessel, fifth conduit means communicating with the top ofsaid absorber, an accumulator, sixth conduit means communicating betweenthe top of said stabilizer vessel and said accumulator, seventh conduitmeans communieating between the top of said accumulator and said fifthconduit means, means for measuring a property of a fluid flowing throughsaid fifth conduit means upstream of said communication between saidseventh and fifth conduit means representative of the compositionthereof, means for forming a first pneumatic pressure signal Crepresentative of said measurement, a first pneumatic computing means,eighth conduit means communicating with said first computing means fortransmitting said pneumatic pressure signal C representative of saidupstream fifth conduit flow measurement from said means for forming tosaid first computing means, a lag means, sixth conduit meanscommunicating between said eighth conduit means and said lag means, asecond pneumatic computing means, tenth conduit means communicatingbetween said lag means and said second computing means for transmittinga pneumatic pressure C to said second computing means, a restrictionmeans, eleventh conduit means communieating between said lag means andsaid restriction means, twelfth conduit means communicating between saidrestriction means and said second computing means for transmitting apneumatic pressure B to said second. computing means, said secondcomputing means solving the equation where g is the adjustable gain ofsaid second computing means, thirteenth conduit means communicatingbetween said second computing means and said first computing means fortransmitting a pneumatic pressure B representative of said Output toAsaid first computing means, means for transmitting a bias pressure A tosaid first computing means, said first computing means solving theequation Output=g(A -C) +B first control means for manipulating the rateof fluid flow through said second conduit means, and fourteenth conduitmeans in communication with said first computing means for transmittinga pneumatic pressure representative of said Output from said firstcomputing means to said first control means to thereby manipulate therate of fluid flow through said second conduit means responsive to saidOutput, second control means for manipulating the rate of fluid flowthrough said seventh conduit means, second means for measuring aproperty of a fluid fiowing through said fifth conduit means downstreamof the communication between said fifth and said seventh conduit meansrepresentative of the composition thereof, and means for forming and fortransmitting a signal from said second means for measuring to said meansfor controlling the rate of fiuid flow through said seventh conduitmeans to thereby manipulate the rate of fiuid fiow through said seventhconduit means responsive to said second means for measuring.

13. Apparatus comprising =an absorber, a stripper vessel, a stabilizervessel, first conduit means in communication with the lower region ofsaid absorber, second conduit means in communication with the upperregion of said absorber, third conduit means communicating between theIbottom of said absorber and said stripper vessel, fourth conduit meanscommunicating between the upper region of said stripper Vessel and saidstabilize-r vessel, fifth conduit means communicating with thetop ofsaid absorber, an accumulator, sixth conduit means communicating betweenthe top of said stabilizer vessel and said accumulator, seventh conduitmeans communicating between the top of said accumulator and said fifthconduit means, means for measuring a property of a fiuid flowing throughsaid fifth conduit means upstream of said communication between saidseventh and fifth conduit mean-s representative of the compositionthereof, means for forming a first pneumatic pressure signal Crepresentative of said measurement, a first pneumatic computing means,eighth lconduit means in communication with said first computing meansfor transmitting said pneumatic pressure signal C representative of saidupstream fifth conduit fiow measurement from said means for forming tosaid first computing means, a lag means, ninth conduit meanscommunicating between said eighth conduit means and said lag means, asecond pneumatic computing means, tenth conduit means communicatingbetween said lag means and said second computing means for transmittinga pneumatic pressure C' to said. second computing means, a restrictionmeans, eleventh conduit means communicating between said lag means andsaid restriction means, twelfth conduit means communicating between said-restriction means and said second computing means for transmitting apneumatic pressure B to said second computing means, thirteenth conduitmeans communicating between said restriction means and Asaid secondcomputing means for transmitting a pneumatic pressure A to said secondcomputing means, said second computingl means being capable of solvingthe equation Output=A-C+B fourteenth conduit means communicating betweensaid second computing mean-s and said first computing means fortransmitting apneumatic pressure A representative of said Output to saidfirst computing means, means for 20 transmitting a bias pressure B tosaid first computing means, said first computing means solving theequation Output=A -C-f-B first control means for manipulating the rateof fluid fiow through said second conduit means, and fifteenth conduitmeans in communication with said first computing means for transmittinga pressure representative of said Output from said first computing means`to said first control means to thereby manipulate the rate of fluidfiow through said second conduit means responsive to said Output, secondcontrol means for manipulating lthe rate of fluid flow through saidseventh conduit means, second means for measuring a property of a yiiuidflowing through said fifth conduit means downstream of the communicationbetween said. fifth and said seventh conduit means representative of thecomposition thereof, and means for forming and for transmitting a signalfrom said second means for measuring to said means for controlling therate of fluid fiow through said seventh conduit means to therebymanipulate the rate of fiuid flow through -said seventh conduit meansresponsive to said second means for measuring.

References Cited by the Examiner UNITED STATES PATENTS 2,600,133 6/1952Simms 196-8 2,771,149 11/1956 Miller et al 48-196 2,996,676 8/1961Shawhan 328-132 3,077,557 2/1963 Joline et al. 340-164 3,088,664 5/1963Oglesby et a1. 23S-200 3,101,433 8/1963 Miller et al 317-149 3,115,44512/1963 Kleiss et al. 196-132 3,158,556 11/1964 Hopper 196-132 3,197,1387/1965 Lupfer 235--200 DELBERT E. GANTZ, Primary Examiner.

H. LEVINE, Assistant Examiner.

3. IN A PROCESS WHICH COMPRISES A RAW NATURAL GAS FEED TO AN ADSORPTIONZONE, PASSING A LEAN ADSORPTION OIL FEED TO SAID ADSORPTION ZONE,WITHDRAWING FROM SAID ADSORPTION ZONE A RESIDUE GAS, AND WITHDRAWINGFROM SAID ADSORPTION ZONE A RICH ADSORPTION OIL CONTAINING NATURALGASOLINE; A METHOD OF CONTROL WHICH COMPRISES MEASURING THE TEMPERATUREOF SAID LEAN ADSORPTION OIL FEED, FORMING AND TRANSMITTING A FIRSTSIGNAL C REPRESENTATIVE OF SAID MEASUREMENT TO A COMPUTING ZONE,APPLYING SAID FIRST SIGNAL TO THE INLET OF A RESTRICTION ZONE TO PRODUCEAT THE OUTLET OF SAID RESTRICTION ZONE A SECOND SIGNAL B, PASSING SAIDSECOND SIGNAL TO SAID COMPUTING ZONE, SAID COMPUTING ZONE SOLVING THEEQUATION